Such a process and such a plant are known from WO 2012/000727 A1. That publication describes especially various connection arrangements of a plurality of membrane units with which a product gas that contains a very high proportion of methane can be achieved. At the same time, the offgas stream is to contain as small an amount of methane as possible. Although it is mentioned in that publication that the pressure difference at the selectively permeable membrane influences the selectivity thereof, no further use is made of that finding. Instead, the method chosen for increasing the selectivity of the process is a specific manner of recycling of gas streams. Permanent monitoring of the quality of the product gas, and in particular regulation to a specific proportion of methane therein, does not take place here.
In EP 1 324 815 B1 it is described, using the example of a plant that produces nitrogen of high purity, that the quality of the product gas can be monitored continuously by a sensor, the pressure difference at at least one membrane unit then being changed accordingly in order to ensure a specific product gas quality. This change takes place in EP 1 324 815 B1 by changing the speed of the compressor which compresses the fed gas mixture, that is to say on the high-pressure side of the membrane, which has the result that the volume flow passing through the plant changes in a readjustment process. This is generally less desirable.